This invention generally relates to the control of temperature in furnaces or kilns for the heat treatment of metals or the firing of ceramics.
Most controllers that are used with furnaces are described as single loop controllers and are of PID type (three mode controllers). PID type controllers get their name from the Proportional, Integral, and Derivative actions that are employed to produce the control action.
The Proportional Action (sensitivity) supplied by the controller is an output or action that is determined from the instantaneous difference (deviation) between the current measured temperature and the desired temperature. The output action is normally limited to the range between zero and 100% and represents a fraction of the rated energy input to the furnace's heating element(s). Most proportional action controllers also operate with an adjustable gain which may be called proportional band control.
Integral Action, also called reset action, is an output or action based on the time integral of the deviation (difference between the desired temperature and the current measured temperature). This too, is usually adjustable and is frequently inhibited during certain occurrences.
Derivative Action, also called rate action, is an output that is proportional to the rate of change of the measured temperature. This action is also adjustable.
The major problem with the use of a three mode controller is that the quality of the results is dependent upon the user's experience or training with control systems in general. Further, the quality of the results is dependent upon the user's experience with the specific furnace, temperature probe and controller being used. Repeated heating tests over a prolonged period may be required to establish control settings and obtain the degree of control required for the application.
This results in costly expenditures for energy and manpower to get the system in operation. Or often times with less experienced or less knowledgeable operators, it results in poor furnace control and some loss in quality of product receiving the heat treatment.
J. G. Zigler and N. B. Nichols in "Optimum Settings for Automatic Controllers," November 1942 Transactions of the A.S.M.E., and in "Process Lags in Automatic-Control Circuits", July 1943 Transactions of the A.S.M.E., teach that the process reaction curve can be determined graphically by supplying a known input and then letting the process measuring device plot the resultant curve. From the curve, one can calculate the system's reaction rate (R) and lag (L). (See FIG. 8, p. 763, Nov. 42.) Although Zigler's and Nichols' graphical methods were demonstrated to work, they are not widely taught or used in thermal process control.
Once the system is tuned and is placed in operation, frequent monitoring of the results may be necessary to see if the system is heating or controlling. Failure of control system components can result in costly overheating or wasted time in getting the system back into operation. Failure to closely monitor the system will also lead the operator to believe the system is doing what has been set or programmed. Thus, the need to retune or change control settings may be overlooked until subsequent tests of the product indicate poor results were achieved.
Therefore, it is desirable to provide a method for accurately controlling the temperature of a furnace. It is also desirable to provide a method for quickly increasing the temperature of a furnace to a final temperature without oscillation or overshoot. It is further desirable to provide control apparatus for performing the above described functions. It is lastly desirable to provide the control apparatus with fail-safe features for terminating operation of the furnace and notifying the user in case of control system malfunction.